Circuit scheme adapted to preclude spurious couplings in radio frequency amplifiers



Apmfi 23, E. ZEPLER CIRCUIT SCHEME ADAPTED TO PRECLUDE SPURIQUSCOUPLINGS IN RADIO FREQUENCY AMPLIFIERS Filed Sept. 27, 1955 INVENTOR ENCH Z E PLEFR ATTORNEY Patented Apr. 23, 1935 UNITED" STATES I CIRCUITSCHEME ADAPTED TO PRECLUDE SPURIOUS COUPLINGS IN RADIO 'FRE- QUENCYAMPLIFIERS Erich Zepler, Berlin, Germanmassignor to TelefunkenGesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, acorporation of Germany Application September 27, 1933, Serial No.691,129 In Germany September 20, 1932 2 Claims. 5 (Cl. 179171) Thepresent invention relates to amplifier circuits and more particularly tomeans for preventing undesired coupling between the various stages ofthe amplifier. An object of the present 5 invention is to, so to speak,separate, as regards radio frequencies, the cathodes or heater wires ofthe various tubes.

In the drawing:-

Fig. 1 illustrates, in diagrammatic form, a circuit arrangement whichwill be referred to hereinafter for purposes of explaining the operationof the invention, and

Fig. 2 is a circuit diagram of certain portions of an amplifier,incorporating the preferred embodiment of the invention.

In the case of receivers having a very high radio frequencyamplification factor the optimum mode of construction is to dispose eachtuned circuit with the tube pertaining thereto within a distinctmetallic box and to join these boxes with one anothersingle-pole-fashion. But such a form of construction is costly forconstructional reasons, and moreover it is unstable in a mechanicalrespect. Instead of using individual boxes a more recommendable plan isto employ a joint casing furnished with partitions. But such a design isimpossible because the shortcircuit currents set up by the coils insidethe casing would enter into mutual coupling relations. It is customaryto accommodate the rotary condensers inside such a metallic casing.While such a construction still satisfies generally present dayrequirements, it is attended with difficulties whenever D. C. separationof the easing in reference to the cathode is demanded.

Fig. 1 illustrates a pair of thermionic tubes, provided respectivelywith tuned circuits l and 2. The common cathode circuit of the two tubesis united to the shielding casing by means of a condenser Ck.

The couplings then arising can be seen from Fig. 1. The grid points ofthe tuning circuits I and 2 (especially the stators of the rotarycondensers) present an appreciable capacitance against the casing. Theensuing capacitive casing currents have their return by way of thegrounding condenser Clc'of the casing, in other words, they thusoccasion capacitive coupling.

Now, according to the present invention the cathodes of the tubes to bebalanced out or neutralized are separated from one another as regardsradio frequencies while each one thereof is separately united with thecasing. (Fig. 2.) Where short waves are concerned, the impedanceresiding in the connection between casing and cathode will suflice tocreate disturbing or spurious couplings; hence, the invention willbecome of added value whenever no D. C. separation of the casing fromthe cathodes is required.

Fig. 2 also illustrates a pair of thermionic tubes, providedrespectively with tuned circuits I and 2. The cathode end of the tunedcircuit I is connected to the end of the shielding casing through thecondenser Ck while the cathode side of the tuned circuit 2 is connectedto the casing through a separate condenser Ck. The cathode circuits ofthe two tubes are united by means of the two impedances shown. It willbe seen from the above that in a system such as shown in Fig. 1, thesingle condenser connection between the cathodes of the two tubes andthe shield causes high frequency currents in one of the circuits to betransferred to the other circuit through the common cathode connection.Thus, in a system such as shown in Fig. 1, a high frequency loop circuitis formed between the grid of the first tube and the shield due to theinherent capacity between the tube circuit and the shield. The remainingportion of the loop circuit comprises the condenser Ck and the cathodeof the tube. The same thing is true of the second tube. Hence, thereresult two loop circuits having a common portion Ck, which means thatthere will be undesired coupling between the two loop circuits. Inaccordance with the present invention, each cathode is connected to theshield through an individual condenser and the common cathode connectionis provided with chokes. Thus, the high frequencies are, in effect,isolated in each circuit, since each circuit forms a complete loopinitself and the chokes act to prevent the transference of energy fromone to the other through the common cathode circuit.

I claim:

1. In a circuit arrangement of the kind described, at least twoelectronic tubes each thereof being provided with input and outputcircuits,

a connection common to the cathodes of said and the capacity connectionbetween the shielding means and the cathode, a radio frequency choke insaid common cathode connection whereby high frequency currents areprevented from being transferred from one device to the other thereofthrough the common cathode connection.

2. In a cascaded radio frequency amplifier a pair of electron dischargetubes each thereof being provided with tuned input circuit, a shieldingmeans for said amplifier, a single cathode circuit for both said tubes,a connection including a condenser between the cathode of one of saidtubes and the shielding means, a connection including a condenserbetween the cathode of the other tube and the shielding means and meansincluding a radio frequency choke coil in the single cathode circuitbetween the cathodes of the two tubes for preventing the transference of.high frequency energy from one of the tuned circuits to the otherthereof through the single cathode circuit.

